For many patients, it is difficult to durably control human immunodeficiency virus type 1 (HIV-1) replication with existing therapies, in part because of the extraordinary capacity of the virus to develop resistance to antiretroviral drugs. All currently available classes of antiretroviral therapy (ART), i.e. protease (PI), reverse transcriptase (RTI), and fusion inhibitors (Fl) can select for mutations within the target gene that confer high- level drug resistance. Indeed, the capacity of a novel compound to select for a mutation is now often used as evidence that the experimental agent has anti-HIV activity. Although these mutations result in an increased ability of the variant to replicate in the presence of drug, they often reduce the in vitro replicative fitness of the virus compared with wild-type strains (in the absence of drug pressure). Many studies have examined the potential relationship of HIV-1 replicative fitness with plasma viral load, drugs resistance and disease progression. Most of these studies, however, have used different methods to measure viral fitness in vitro, which hamper comparisons of fitness values obtained in different studies. Thus, it is evident that a consensus on the experimental approach used to measure HIV-1 replicative fitness has to be established to allow a direct qualitative and quantitative comparison between different studies. Which (if any) in vitro assay of viral fitness is the most appropriate for comparison with in vivo HIV-1 fitness? By definition, in vitro viral replicative fitness is best measured in growth-competition experiments. However, in vitro estimates of fitness cannot be easily extrapolated to in vivo situations, since actual replication in target cells may be influenced by various host factors and extrinsic interventions such as antiretroviral treatments. In this proposal, we intend to establish a validated protocol to measure HIV-1 replicative fitness in vitro, which could lead to a better understanding of changes in the replicative capacity of the virus upon emergence of drug resistance in vivo. Using this in vitro approach, we will characterize the contribution of drug-resistant associated mutations in the polymerase (po/) and envelope (env) genes to HIV-1 replicative fitness, and correlate this viral parameter with markers of disease progression such plasma HIV RNA. CD4* T-cell count. and antiretroviral treatment. We believe that viral replicative fitness may be playing a bigger role in HIV-1 pathogenesis than recognized to date. Therefore, our proposal may contribute to the development of further studies aimed to understand the biology of the virus/drug-resistance/host immunity interaction, which could lead to the design of novel antiretroviral strategies. RELEVANCE TO PUBLIC HEALTH (lay statement): Viruses that are not able to grow well (with low replicative fitness) may be beneficial to HIV-infected individuals. Thus, measuring the capacity of HIV to replicate may eventually become a standard tool in the management of patients with HIV. For example, it may help guide decisions regarding delaying, initiating or discontinuing antiretroviral therapy. [unreadable] [unreadable] [unreadable]